How to Recognise Japanese Knotweed: The Complete Guide

Japanese knotweed is an invasive plant that can cause severe damage to property in the UK. Learning how to identify this problematic weed is an essential skill for any homeowner or gardener. This comprehensive guide will teach you how to recognise Japanese knotweed during every season and growth stage.

What is Japanese Knotweed?

Japanese knotweed (Fallopia japonica) is a perennial herbaceous plant that originated in Japan, It was introduced to the UK in the 19th century as an ornamental plant but has since become a problematic invasive species

The plant spreads aggressively, growing through cracks in concrete, brickwork, and tarmac. The roots can burrow deep underground, spreading up to 20 metres horizontally and 3 metres vertically. This makes Japanese knotweed incredibly difficult to eradicate fully.

Even small fragments of the plant can regenerate into new growths Any part of the plant, including stems, leaves and roots, can begin rapid growth when dropped or buried elsewhere This vegetative reproduction is the primary way the weed spreads.

Japanese knotweed is very resilient and can push through solid surfaces, causing structural damage. It’s illegal to allow Japanese knotweed to spread onto neighbouring properties in the UK.

Why Identify Japanese Knotweed

There are several crucial reasons you need to learn how to identify Japanese knotweed:

• Prevent Spread – Spotting an infestation early allows you to treat it before it spreads and causes severe damage

• Mortgages – Many lenders will refuse to offer a mortgage if a property has Japanese knotweed present, due to the potential damage it can cause.

• Legal Obligations – You have a legal duty to prevent Japanese knotweed on your land from spreading onto neighbouring properties. Failing to control knotweed can lead to lawsuits and fines.

• Property Values – Japanese knotweed can seriously degrade the value of land and property. Identifying it early maximises the chances of successful elimination.

How to Identify Japanese Knotweed

Japanese knotweed looks different during each growing season. To reliably identify Japanese knotweed, you need to know its key features and appearance throughout the year.

Spring

In spring, Japanese knotweed begins growing bright red or purple asparagus-like spears. These young shoots are sometimes mistaken for rhubarb.

Key springtime identifiers:

• Red or purple shoots emerging from the ground
• Rolled up leaves with red veins
• Hollow bamboo-like stems
• Can grow up to 10cm per day

Summer

Summer is when Japanese knotweed is most visible and easiest to identify. The plant grows rapidly, forming dense thickets of green foliage.

Summer identifiers:

• Distinctive heart or spade-shaped leaves
• Stems turn green with purple flecks
• Alternate leaf arrangement along zig-zag stems
• Grows up to 2-3m tall
• Hollow bamboo-like stems

Late Summer

In late summer, tiny creamy-white flowers start to emerge from the leaf axils. This is a clear confirmation you have spotted Japanese knotweed.

Late summer identifiers:

• Clusters of small white flowers
• Flowers can reach 10cm long
• Flowers amidst leaf foliage
• Size peaks at 2-3m high

Autumn

As autumn approaches, the leaves start to turn yellow and fall. The stems turn brown and form dried, hollow canes.

Autumn identifiers:

• Leaves turning yellow and falling
• Stems turning brown
• Dead hollow canes remain standing
• Dry remains of flowers sometimes still visible

Winter

In winter, the above-ground foliage dies back entirely, leaving nothing but hollow brown canes and stems. Don’t be fooled though, the plant is very much alive underground.

Winter identifiers:

• No leaves present
• Dead hollow stems remain
• Dry, crispy flowers sometimes remain
• Distinctive zig-zag stem pattern still visible

Japanese Knotweed Lookalikes

Several common plants are often mistaken for Japanese knotweed. Here are a few plants that share similar features, and how to tell them apart:

Lesser Knotweed

Lesser knotweed can be identified by its pink flowers and thinner, unbranched stems. The leaves are longer and narrower than Japanese knotweed.

Lilac

Lilac can seem similar from afar. Look for the oppositely arranged leaves and purple flowers to separate it from knotweed.

Bindweed

Bindweed is a creeping vine, unlike Japanese knotweed’s tall, hollow stems. It has trumpet-shaped white flowers and thinner arrowhead-shaped leaves.

Bamboo

Bamboo shares Japanese knotweed’s node-segmented hollow stems. But bamboo has very different drooping leaves that can reach 60cm long.

Himalayan Balsam

This invader has pink-veined, slender leaves rather than Japanese knotweed’s spade shape. It also has distinctive helmet-shaped pink flowers.

Russian Vine

Russian vine can grow rapidly like knotweed. But it’s a thinner climbing vine that lacks Japanese knotweed’s distinctive zig-zag stem pattern.

Parts of a Japanese Knotweed Plant

To identify Japanese knotweed during any part of its lifecycle, you need to recognise these key components:

Stems – Green and hollow with distinct purple speckles. Segmented with nodes like bamboo. Grow in a zig-zag pattern up to 3m high.

Leaves – Spade or heart-shaped with pointed tips. Alternate arrangement along stem. Bright green and up to 14cm long.

Flowers – Numerous tiny creamy-white flowers emerging from spikes up to 10cm long near end of summer.

Roots – Extensive underground network of rhizomes spreading up to 20m horizontally. Dark brown exterior, bright orange inside when cut.

Seeds – Rarely found in the UK. Small, winged, triangular. Do not require seeds to spread – any plant part can generate new growth.

Whole Plant Changes Through the Season

Here is a quick visual guide to how the entire Japanese knotweed plant transforms during spring, summer, autumn and winter:

Spring

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• Reddish asparagus-like shoots emerge from the ground
• Rolled up leaves with red veins
• Hollow stems with purple flecks

Summer

[image]

• Dense thickets of green foliage
• Distinctive spade shaped leaves
• Grows rapidly up to 2-3m high

Autumn

[image]

• Leaves turn yellow and fall off
• Stems turn brown
• Dead flowers remain

Winter

[image]

• No leaves remain
• Only dead hollow canes remain
• Distinguished zig-zag stem pattern

Life Cycle and Growth Rate

Japanese knotweed spreads during spring and summer through rapid growth of new stems from roots and rhizomes. It can grow up to 10cm per day during the peak of summer.

The plant blooms in late summer, producing small white flowers. It begins dying back in autumn as leaves fall and stems turn brown. In winter, it survives underground as rhizomes until returning the following spring.

New plants can sprout from the tiniest 0.7g rhizome fragment during any part of this cycle. The plant also spreads by water distribution. It can survive a wide range of conditions and is extremely difficult to kill fully.

How to Spot Early Growth

The key to effective treatment is identifying Japanese knotweed early in the growing season. Here are tips for spotting young knotweed growth:

• Inspect any red or purple asparagus-like shoots in spring. Japanese knotweed shoots first emerge from the ground in springtime.

• Look for rolled up leaves with reddish veins. The young unfolding leaves first emerge rolled up with a reddish tinge.

• Monitor any rapidly sprouting growth near standing dead canes. Hollow canes from last year reveal where new growth emerges.

• Check areas near water sources like streams. The plant spreads downstream by fragments washing away.

• Follow zig-zag stem patterns down to the ground. New growth often sprouts from last year’s fallen stems.

• Notice if shoots snap easily when bent. The hollow young stems are brittle and prone to snapping.

Catching Japanese knotweed early gives you the best chance of successful treatment before the weed causes serious damage.

What To Do If You Spot Japanese Knotweed

If you suspect you have spotted Japanese knotweed, there are a few important steps to take:

• Identify – Confirm it is Japanese knotweed by matching several identifiable features, or submit photos to experts.

• Assess Extent – Determine the size and areas impacted by the infestation. Roots could spread much further than visible foliage.

• Report – Tell the Environment Agency using their online reporting form for invasive species.

• Treat Rapidly – Arrange accredited professional treatment like herbicide or excavation to control the spread.

• Monitor Site – Check regularly for any regrowth and repeat treatment if required. Maintain a knotweed management plan.

Prevent Spread To Neighbours

You have a legal duty to prevent Japanese knotweed on your land from spreading onto neighbouring properties. Follow these tips to avoid allowing spread:

• Identify the weed early before it can spread. Monitor areas near boundaries vigilantly.

• Arrange immediate professional treatment. Leaving knotweed uncontrolled on a boundary often leads to lawsuits.

• Always properly dispose of any stems, leaves or soil contaminated with Japanese knotweed. Bag securely.

• Never dump any Japanese knotweed waste or plant parts into waterways or other land. This causes further spread.

• Check on-site waste movements and ensure no parts can be lost across boundaries during removal work.

• Ask your neighbours to notify you if any Japanese knotweed appears near shared boundaries.

Preventing Japanese knotweed from invading neighbouring land is a legal requirement for any UK landowner. Doing so minimises the damage and avoids potentially hefty fines or lawsuits.

Legislation and Regulations

There are several laws surrounding Japanese knotweed due to the damage it causes:

• Wildlife and Countryside Act 1981 – makes it illegal to plant or allow Japanese knotweed to spread in wild areas.

• Environmental Protection Act 1990 – classifies Japanese knotweed as controlled waste requiring safe disposal.

• Anti-Social Behaviour Act 2003 – gives councils power to serve treatment notices for failing to control knotweed spread.

• Town and Country Planning Act 1990 – can prosecute for allowing Japanese knotweed to damage the built environment.

Failure to control the spread of Japanese knotweed can lead to five-figure fines or even prison sentences up to 2 years. The law takes this invasive plant very seriously.

Japanese knotweed can be difficult to identify definitively, especially for the untrained eye. However, by learning its key features through the seasons and how to spot early growth, you can reliably recognise an infestation on your property.

Follow the guidance provided in this guide to check for the critical identifiers such as zig-zag stems, spade-shaped leaves, rapid growth and the presence of canes. Seek help from professional knotweed specialists if you are still uncertain.

Catching this destructive invasive plant in the early stages minimises the impact on your land and property. Don’t allow Japanese knotweed to go unchecked – identify it swiftly and take control with effective treatment. With vigilance and care, you can avoid the headaches this damaging plant can cause.